(1) Field of the Invention
The present invention relates to a digital cordless telephone device which gives a warning to prevent its communication with another telephone device from being unexpectedly terminated.
(2) Related Art
Digital cordless telephone devices allow their users to communicate with each other via public base stations which are installed outdoors as well as via private base stations. Such digital cordless telephone devices are standardized under Second Generation Cordless Telephone System RCR STD-28 which has been issued by the Research and Development Center for Radio Systems.
Each base station has its own service area within which users can use digital cordless phones or car phones via the base station. Since users cannot communicate via a base station outside its service area, it is necessary for them to make sure that they are within the service area of a base station to make a telephone call.
Japanese Laid-open Patent Application No. 4-25233 has suggested a digital cordless telephone device which displays a sign to inform the user that the telephone device is not located within the radio zone of a base station. In this technique, a personal station measures a Received-Signal-Strength Indicator (hereinafter RSSI) which is transmitted from the base station, and displays the sign when the measured RSSI is lower than a predetermined level.
The above-mentioned RCR STD-28 defines the hand-over level (Lh) of each base station, and stipulates that each personal station within the service area of a base station should obtain the hand-over level (Lh) of the base station when it starts to connect with a base station or to execute a hand-over.
A hand-over refers to a personal station switching from the current base station to another base station as it moves. To be more specific, a personal station disconnects from the current base station, and searches for other available base stations whose RSSIs are higher than a predetermined threshold by examining the RSSIs of their control channels. Then, the personal station selects the base station that has the highest RSSI among the searched base stations, and executes a hand-over. A hand-over level (Lh) refers to an RSSI level which requires a personal station to execute a hand-over.
FIG. 1 is an illustration to explain how a hand-over is executed with conventional digital cordless telephone devices. The illustration includes base stations 1, 2 and 3, their respective service areas 4, 5, and 6, a mobile personal station 7, and a radio zone 8 of the base station 1. In the radio zone 8, personal stations can receive radio waves from the base station 1.
It is assumed that the mobile personal station 7 is communicating with another personal station via the base station 1, while moving in the direction of the arrow A. The mobile personal station 7 must have obtained the hand-over level (Lh) of the base station 1 when it started the communication. The mobile personal station 7 can continue the communication via the base station 1 as long as it is within the service area 4, which is to say as long as the RSSI of the base station 1 is equal to or higher than the hand-over level (Lh). If the mobile personal station 7 moves across the point P1 on the limit of the service area 4, the RSSI of the base station 1 becomes lower than its hand-over level (Lh). As the result, the mobile personal station 7 executes a hand-over to switch from the base station 1 to the base station 2, thereby continuing the communication.
However, conventional digital cordless telephone devices, such as the mobile personal station 7, have the following problem which will be explained with reference to FIG. 1.
It is assumed that the mobile personal station 7 is communicating with another personal station via the base station 1, while moving in the direction of the arrow B. The RSSI of the base station 1 becomes lower than its hand-over level (Lh) when the mobile personal station 7 goes over point P2. At this point, although the mobile personal station 7 tries to execute a hand-over, there is no base station available. The mobile personal station 7 then tries to re-connect with the base station 1. However, the mobile personal station 7 cannot re-connect with the base station 1 once it gets out of the service area. As a result, the communication is unexpectedly terminated.
To solve this problem, it would be possible for a personal station to give a warning depending on the hand-over level (Lh) of each base station. However, it would cause another problem in that the warning is given even when there is a base station available to execute a hand-over. For example, when the mobile personal station 7 is moving in the direction of the arrow A, a warning would be given when the mobile personal station 7 passes the point Pl, although a hand-over is executed between the base station 1 and the base station 2.
In view of the foregoing problems, the object of the present invention is to provide a digital cordless telephone device which gives a warning to inform the user that it is located in the vicinity of the service area limit of the base station to which it is connected.
The above object can be achieved by a digital cordless telephone device which performs radio communication with another telephone device via one of a plurality of base stations and executes a hand-over to switch its connection from a first base station to a second base station based on an RSSI of the first base station, including the following: an RSSI detection unit for detecting the RSSI of the first base station; a comparison unit for comparing the RSSI detected by the RSSI detection unit with a predetermined threshold; a connection control unit for trying to execute a hand-over when the RSSI is smaller than the predetermined threshold; a measurement unit for counting a number of continuous hand-over failures; and an information unit for giving a warning to a user of the digital cordless telephone device when the number counted by the measurement unit has reached the predetermined threshold.
In the above-explained construction, when the digital cordless telephone device has failed in hand-overs for a predetermined number of times on end, it gives a warning to inform the user that the digital cordless telephone device is located near the service area limit and that it is highly possible that the communication is terminated. Consequently, the user can stop moving further or move in a different direction, thereby preventing the communication from being terminated.
The above object can be also achieved by another digital cordless telephone device which includes the following: an RSSI detection unit for detecting RSSIs of the plurality of base stations; a comparison unit for comparing an RSSI of the first base station which has been detected by the RSSI detection unit with a predetermined threshold; a search unit for searching for all base stations that can be a target station for re-connection when the RSSI of the first base station is smaller than the predetermined threshold, and for selecting one base station among all the base stations; a measurement unit for counting a number of times the search unit has selected the first base station continuously; and an information unit for giving a warning to a user of the digital cordless telephone device when the number counted by the measurement unit has reached the predetermined threshold.
The connection control unit may include the following: a disconnection unit for disconnecting the digital cordless telephone device from the first base station when the RSSI is smaller than the predetermined threshold; a search unit for searching for the second base station when the disconnection unit has disconnected the cordless telephone device from the first base station; a hand-over unit for executing a hand-over when the search unit has searched for the second base station; a re-connection unit for trying to re-connect the digital cordless telephone device with the first base station, when the second base station does not exist; and a judgement unit for judging a hand-over to have ended in failure when the re-connection unit has successfully re-connected the digital cordless telephone device with the first base station.
The judgement unit may include the following: a first judgement unit for judging whether the re-connection unit has successfully re-connected the digital cordless telephone device with the first base station; and a second judgement unit for judging whether the RSSI of the first base station is smaller than the predetermined threshold when the first judgement unit has judged that the digital cordless telephone device has been successfully re-connected with the first base station. The judgement unit judges a hand-over to have ended in failure when the second judgement unit has judged the RSSI of the first base station to be smaller than the predetermined threshold.
The measurement unit may include the following: a counter for counting a number of hand-over failures which have been judged by the judgement unit. The measurement unit resets the counter when the hand-over unit has successfully executed a hand-over and when the second judgement unit has judged the RSSI of the first base station to be one of being equal to and larger than the predetermined threshold.
The search unit may include the following: a detection unit for detecting RSSIs of the plurality of base stations except the first base station; and a judgement unit for judging a presence or absence of the second base station from the RSSIs detected by the detection unit.
The comparison unit may include a storage unit for storing the predetermined threshold.
The digital cordless telephone device may further include a reception unit for receiving the predetermined threshold from the first base station. The storage unit stores the predetermined threshold received by the reception unit.
The information unit may sound a beep as the warning through a speaker.
The information unit may sound the beep when the number of hand-over failures counted by the measurement unit has become two.
The measurement may include a counter for counting a number of times the search unit has selected the first base station. The measurement unit resets the counter when base stations other than the first base station have been selected.